Editorial Feature

An Introduction to Photo-Induced Force Microscopy (PIFM)

There are many types of microscopy in the marketplace today, and some are now widely used across all scientific branches. A few prominent examples are optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

These techniques allow us to “see” the physical structure of objects at the micro- and nanoscale.  Adding spectroscopic capability to some of these techniques can provide the ability to identify materials at the nanoscale.  For example, adding energy dispersive X-ray (EDX) spectroscopy to an electron microscope is a widely used method to provide identification of elemental materials at the nanoscale.

Adding optical spectroscopy to AFM now offers a similar promise of identification of molecular materials at the nanoscale.  One particularly promising case is photo-induced force microscopy (PiFM), which combines optical absorption spectroscopy with AFM.  In this article, we look into this emerging technique.

As it is a relatively new technique, PiFM is not known by many scientists. In short, along with imaging topography, PiFM provides chemical imaging as well.  PiFM does this by measuring optical near-fields in nanoscale structures. The spectroscopic information generated by PiFM has a spatiotemporal resolution of less than 10 nm and broadband spectral sensitivity.

A PiFM instrument starts as a complete AFM, to which is added a broadly tunable laser and the necessary optics to focus the light from this laser on the tip-sample interface.  As the laser light excites material-specific polarization within various nanofeatures of the sample, forces are exerted on the metalized AFM tip, and by very sensitively mapping these forces as a function of wavelength and position, specific spectra are generated corresponding to the various nanostructures that are visible.

These spectra correlate with known optical absorption spectra (for example, those obtained by conventional FTIR) and can therefore be used to identify specific materials in individual nanostructures.  While the AFM tip needs to be very close (within a few nm) to the sample to detect the forces of interest, tip-sample contact is not necessary, allowing PiFM to be a nondestructive, noncontact technique.

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PiFM isn’t the first technique to combine AFM with focused light, but it has unique advantages in that it can operate with little or no tip-sample contact (unlike similar techniques that sense physical thermal expansion of the sample) and relies on detecting mechanical force on the tip instead of collecting scattered light.

This makes PiFM a naturally very high resolution technique – routinely showing 10nm resolution or better and gives it an inherently high signal-to-noise ratio.  PiFM detects the photo-induced molecular polarizability at the molecular level by mechanically detecting the force gradient arising from the interaction between the optically driven molecular dipole and its mirror image in the metallized tip.

Neither the thickness nor the thermal behavior of the sample play a significant role in PiFM, so PiFM places few constraints on the types of samples that can be studied.  Since the tip-sample forces responsible for PiFM have an extremely steep distance dependence, PiFM is a highly surface-sensitive technique.

PiFM is surprisingly user-friendly, unlike earlier AFM-based optical spectroscopy techniques (for example, tip-enhanced Raman spectroscopy). PiFM uses low-cost, widely available commercial tips, and getting good results doesn’t depend on selecting “perfect” tips that are few and far between; most tips give good results.

Currently available PiFM systems use a pulsed IR quantum cascade laser (QCL) as the tunable light source.  The range of wavelengths available from a QCL provides access to the “molecular fingerprint” region of the IR absorption spectrum, which is why PiFM works so well for identifying molecular materials such as polymers and a wide variety of organic and inorganic materials.

In addition, it is useful for examining plasmonic behavior of nanostructures and certain kinds of nanoscale physical effects such as polaritons.  PiFM isn’t limited to the IR spectral range.  It has already been shown to work well in the visible range as well, and likely is extendable over a much wider spectral range when coupled to appropriate light sources.

Its compatibility with a wide variety of sample materials, coupled with its friendly attributes (compared to competing techniques), has already allowed PiFM to establish itself as an attractive method for the visualization and spectroscopic characterization of nanomaterials.

At the moment, this ranges from semi-conducting nanoparticles, to polymer thin films, to sensitive measurements of single molecules, but will no doubt increase in the future as the techniques becomes more widely known and widely used across varying applications.

 

Sources:

Molecular Vista- http://molecularvista.com/technology/pifm/

University of Michigan- https://events.umich.edu/event/34588

“Photo-induced force for spectroscopic imaging at the nanoscale”- Jahng J., et al, Proc. SPIE 9764, Complex Light and Optical Forces X, 97641J, 2016

“Photoinduced Force Mapping of Plasmonic Nanostructures”- Tumkur T. U., et al, ACS Nanoletters, 2016

https://molecular-plasmonics.de/molplasmon17/pdf/abstracts/Meyer.pdf

Image Credit: https://www.youtube.com/watch?v=xrnYImhxeT4https://www.youtube.com/watch?v=xrnYImhxeT4

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